Coating formulations are commonly applied to a substrate by passing the coating formulation under pressure through an orifice into air in order to form a liquid spray, which impacts the substrate and forms a liquid coating. In the coatings industry, three types of orifice sprays are commonly used; namely, air spray, airless spray, and air-assisted airless spray.
Air spray uses compressed air to break up the liquid coating formulation into droplets and to propel the droplets to the substrate. The most common type of air nozzle mixes the coating formulation and high-velocity air outside of the nozzle to cause atomization. Auxiliary air streams are used to modify the shape of the spray. The coating formulation flows through the liquid orifice in the spray nozzle with relatively little pressure drop. Siphon or pressure feed, usually at pressures less than 18 psi, are used, depending upon the viscosity and quantity of coating formulation to be sprayed.
Airless spray uses a high pressure drop across the orifice to propel the coating formulation through the orifice at high velocity. Upon exiting the orifice, the high-velocity liquid breaks up into droplets and disperses into the air to form a liquid spray. Sufficient momentum remains after atomization to carry the droplets to the substrate. The spray tip is contoured to modify the shape of the liquid spray, which is usually a round or elliptical cone or a flat fan. Turbulence promoters are sometimes inserted into the spray nozzle to aid atomization. Spray pressures typically range from 700 to 5000 psi. The pressure required increases with fluid viscosity.
Air-assisted airless spray combines features of air spray and airless spray. It uses both compressed air and high pressure drop across the orifice to atomize the coating formulation and to shape the liquid spray, typically under milder conditions than each type of atomization is generated by itself. Generally the compressed air pressure and the air flow rate are lower than for air spray. Generally the liquid pressure drop is lower than for airless spray, but higher than for air spray. Liquid spray pressures typically range from 200 to 800 psi. The pressure required increases with fluid viscosity.
Air spray, airless spray, and air-assisted airless spray can also be used with the liquid coating formulation heated or with the air heated or with both heated. Heating reduces the viscosity of the liquid coating formulation and aids atomization.
U.S. Pat. Nos. 3,556,411; 3,647,147; 3,659,787; 3,754,710; 4,097,000; and 4,346,849 disclose spray nozzles and tips for use in airless spray, including designs and methods of manufacture and methods of promoting turbulence in the atomizing fluid. U.S. Pat. Nos. 3,907,202 and 4,055,300 disclose spray nozzles for use in air-assisted airless spray. None of these patents uses supercritical fluids as diluents to spray coating formulations.
More information about orifice sprays such as air spray, airless spray, and air-assisted airless spray and about heated sprays can be obtained from the general literature of the coating industry and from technical bulletins issued by spray equipment manufacturers, such as the following references:
1. Martens, C. R., Editor. 1974. Technology of Paints, Varnishes and Lacquers. Chapter 36. Application. Robert E. Krieger Publishing Company, Huntington, N.Y. PA0 2. Fair, James. 1983. Sprays. Pages 466-483 in Grayson, M., Editor. Kirk-Othmer Encyclopedia of Chemical Technology. Third Edition. Volume 21. Wiley-Interscience, New York. PA0 3. Zinc, S. C. 1979. Coating Processes. Pages 386-426 in Grayson, M. Editor. Kirk-Othmer Encyclopedia of Chemical Technology. Third Edition. Volume 6. Wiley-Interscience, New York. PA0 4. Long, G. E. 1978 (March 13). Spraying Theory and Practice. Chemical Engineering: 73-77. PA0 5. Technical Bulletin. Air Spray Manual. TD10-2R. Binks Manufacturing Company, Franklin Park, Ill. PA0 6. Technical Bulletin. Compressed Air Spray Gun Principles. TD10-1R-4. Binks Manufacturing Company, Franklin Park, Ill. PA0 7. Technical Bulletin. Airless Spray Manual. TD11-2R. Binks Manufacturing Company, Franklin Park, Ill. PA0 8. Technical Bulletin. Airless Spraying. TD11-1R-2. Binks Manufacturing Company, Franklin Park, Ill. PA0 9. Technical Bulletin. Hot Spraying. TD42-1R-2. Binks Manufacturing Company, Franklin Park, Ill. PA0 10. Technical bulletin on air-assisted airless spray painting system. Kremlin, Incorporated, Addison, Ill. PA0 (1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA0 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon. PA0 (1) means for supplying at least one polymeric compound capable of forming a continuous, adherent coating; PA0 (2) means for supplying at least one active organic solvent; PA0 (3) means for supplying supercritical carbon dioxide fluid; PA0 (4) means for forming a liquid mixture of components supplied from (1)-(3); PA0 (5) means for spraying said liquid mixture onto a substrate. PA0 (1) forming a liquid mixture in a closed system, said liquid mixture comprising: PA0 (2) spraying said liquid mixture onto a substrate to form a liquid coating thereon by passing the mixture under pressure through an orifice into the environment of the substrate to form a liquid spray.
Prior to the present invention, the liquid spray application of coatings, such as lacquers, enamels, and varnishes, by the spray methods discussed above was effected solely through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution resulting from painting and finishing operations. For this reason there is great need for new liquid spray technology for application of coatings that diminishes the emission of organic solvent vapors.
U.S. Pat. No. 4,582,731 (Smith) discloses a method and apparatus for the deposition of thin films and the formation of powder coatings through the molecular spray of solutes dissolved in organic and supercritical fluid solvents. The molecular sprays disclosed in the Smith patent are composed of droplets having diameters of about 30 Angstroms. These droplets are more than 10.sup.6 to 10.sup.9 less massive than the droplets formed in conventional application methods that Smith refers to as "liquid spray" applications. Furthermore, the orifice used to produce the molecular sprays is typically in the 1 to 4 micron diameter size range. These orifice sizes are 10.sup.3 to 10.sup.5 times smaller in area than orifices used in conventional "liquid spray" apparatus. This disclosed method of depositing thin films seeks to minimize, and preferably eliminate, the presence of solvent within the film deposited upon a substrate. This result is preferably accomplished through the maintenance of reduced pressure in the spray environment. However, the maintenance of reduced pressures is not feasible for most commercial coating applications. Furthermore, the spray method disclosed by Smith utilizes very high solvent-to-solute ratios, thereby requiring undesirably high solvent usage and requiring prohibitively long application times in order to achieve coatings having sufficient thicknesses to impart the desired durability of the coating.
U.S. Patent Application Ser. No. 133,068, filed Dec. 21, 1987, (Hoy et al) discloses a process and apparatus for the liquid spray application of coatings to a substrate wherein the use of environmentally undesirable organic diluents is minimized. The process of the invention comprises:
(a) at least one polymeric compound capable of forming a coating on a substrate; and PA1 (b) at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture of (a) and (b) to a point suitable for spray applications; PA1 (a) at least one polymeric component capable of forming a coating on a substrate; and PA1 (b) a solvent component containing at least one supercritical fluid, in at least an amount which when added to (a) is sufficient to render the viscosity of said mixture to a point suitable for spray application;
The invention is also directed to a liquid spray process as described immediately above to which at least one active organic solvent (c) is admixed with (a) and (b), prior to the liquid spray application of the resulting mixture to a substrate. The preferred supercritical fluid is supercritical carbon dioxide fluid. The apparatus of the invention comprises an apparatus in which the mixture of the components of the liquid spray mixture can be blended and sprayed onto an appropriate substrate. Said apparatus is comprised of, in combination:
The apparatus further comprises (6) means for heating any of said components and/or said liquid mixture of components. Hoy et al demonstrate the use of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute these compositions to application viscosity required for liquid spray techniques. They further demonstrate that the method is generally applicable to all organic solvent borne coatings systems. However, they do not teach the means for spraying.
Supercritical carbon dioxide fluid is an environmentally safe, non-polluting diluent that allows utilization of the best aspects of organic solvent borne coatings applications and performance while reducing the environmental concerns to an acceptable level. It allows the requirements of shop-applied and field-applied liquid spray coatings as well as factory-applied finishes to be met and still be in compliance with environmental regulations.
Clearly what is needed is a liquid spray method of coating substrates that can be applied to using supercritical fluids, such as supercritical carbon dioxide fluid, as diluents to reduce coating formulations to spray viscosity. Such a method should utilize the properties of the supercritical fluid, should be compatible with existing spray technology and practice, and should be environmentally acceptable.
Prior to the present invention, however, it was unknown how a high concentration of highly volatile supercritical fluid, such as supercritical carbon dioxide fluid, would affect formation of a polymeric liquid spray. A spray mixture undergoes a large and rapid drop in pressure as it goes through the orifice. Accordingly, it was theorized that the supercritical spray mixture would produce a foam like shaving cream instead of a spray, because nucleation to form gas bubbles would be so rapid and intense. Alternatively, it was expected that the spray mixture would produce a mist or fog of microdroplets instead of a spray, because atomization would be so intense. Another likely anticipated result was that the spray mixture would produce a spray of bubbles instead of droplets. Furthermore, even if a spray were formed, it would have been expected that sudden and intense cooling that accompanies rapid depressurization and expansion of a supercritical fluid would cause the liquid droplets to freeze solid. For example, it is commonly known that the spray from carbon dioxide fire extinguishers produces solid dry ice particles.
In the event that formation of a liquid spray were achieved, it was further considered unlikely that the spray could be used to produce quality coherent polymeric coatings on a substrate. It would be surmised that the liquid droplets would be so small or have so little momentum that they could not be deposited well onto the substrate. It was expected that foaming droplets or supercritical fluid dissolved in the coating would produce a layer of foam on the substrate or a coating full of bubbles when these characteristics were not desired in the coating. The liquid coating droplets that are deposited onto the substrate would have a much higher viscosity than the material that was sprayed, because they would have lost most of the supercritical fluid diluent and they would be at a lower temperature. Furthermore, the coating material would contain less volatile organic solvent than normal. Therefore, it was expected that higher viscosity would prevent or hinder coalescence of the deposited droplets to form a coherent liquid coating; that it would reduce how much the droplets spread out on the substrate, so that thin coatings could not be produced; and that it would reduce the surface flow that produces a smooth coating. It was further expected that moisture would condense onto the droplets and harm the coating, because the spray would be cooled below the dew point.
Surprisingly, however, it has been discovered that liquid sprays can be formed by using supercritical fluids as viscosity reduction diluents and that such sprays can be used to deposit quality coherent polymeric coatings onto substrates.
It is accordingly an object of the present invention to demonstrate the use of orifice sprays, such as airless spray and air-assisted airless spray, to apply liquid coatings to substrates by liquid sprays in which supercritical fluids, such as supercritical carbon dioxide fluid, are used as diluents in highly viscous organic solvent borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute these compositions to application viscosity.
A further object of the invention is to demonstrate that the method is generally applicable to all organic solvent borne coatings systems.
These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.